The horse's "engine" is always in the rear. When the horse is in ventroflexion with the head pulled up, it sort of stops the flow of the power from the hindquarters and the horse ends up pulling with the front end.
The show horses, and any other horse that is ridden in that style, will have hypertrophy of the shoulder muscles, and weak hindquarters.
We probably do not want to learn from trainers who ride the horse's face.
Here's an excerpt from Horse Breeding and Management about hypertrophy.
1. Hypertrophy due to poor overall balance or lack of self-carriage. This is a problem of green and badly-trained horses. Muscles typically affected are the serrati of both the neck and body and the pectorals, especially the flat portions of the muscle that lie between the arms and that extend directly from the arms to the sternum just in front of the girth. The muscles will be too firm in tone and bulging.
Rigidity in the serrati and pectoral muscles is the primary cause of binding up or restriction of movement in the shoulder.
Dressage was invented in order to produce self-carriage and loose, mobile shoulders; its exercises, when correctly executed, have the effect of inducing the horse to cease to carry itself on the forehand or to stretch the muscles of the neck, back, and shoulders (d la Gueriniere, 1751).
2. Hypertrophy due to incorrect dressage. Over-developments and muscular rigidities of the latissimus and rhomboideus muscles are especially prevalent among horses engaged in the new, competitive form of dressage.
The cure for these problems is to stop pushing the horse onto the forehand during extensions, stop permitting or encouraging it to hollow its back and "flick" or hyper extend the forelimb, stop hanging on its mouth and go back to insisting that back and neck be swinging and elastic and that the fore and hind strides be of equal size during all phases of the trot (Hebermann, 1984).
3. Hypertrophy due to the application of weights to the forelimbs. There is almost no muscular tissue in the forelimb of the horse below the carpus or knee. The bulk of muscles that control the swing of the forelimbs is located above the elbow.
The majority of trainers who use weights such as heavy shoes or weighted boots on horses believe that their purpose is to product motion. In fact, weights were originally used to change the timing of the footfalls which they do with an effect similar to that produced by adding weights to cogwheels or camshafts. The major side-effect of weights is to exponentially increase the amount of lateral instability (wobble) in the forelimb of the horse. When the weights are over 12 oz., forced hyperextension of the shoulder joint also occurs.
Under normal circumstances, lateral instability of the limb is controlled primarily by the pectoral muscles while movements of the scapulohumeral joint are controlled by the infraspinatus, supraspinatus, and subscapularis muscles.
The the horse, the arm is attached to the shoulder blade by no ligaments at all, except by the paper-thin joint-capsule. Therefore, under the influence of weights, all muscles spanning the shoulder joint hypertrophy, pushing the scapulas laterally and producing a horse with an abnormally wide chest.
Heavy weights also induce hypertrophy of the brachiocephaicus and trapezius muscles from which an abnormally rigid neck carriage results (Bennett, 1984).
Since the use of weights is entirely directed to forcing the horse to produce artificial or fantasy gaits often resulting in joint injuries, self-defensive muscular hypertrophy and rigidity in the horse, their use cannot be justified.
Here's a couple of older posts where we were discussing these issues: